The popular Hindu theory of the transmigration of souls is not quite the same as the philosophic theory of the evolution of souls. According to the first, a man may once again become an animal or a tree; according to the second, this is not part of the ordinary processes of Nature. Many superstitions, however, hide some truth among their nonsense, and this is one of them. Just as every biologist knows that Nature sometimes produces freaks, and every physician knows that monstrosities are sometimes born into the human race, so there are cases where a deranged mind frantically thirsting for a physical body after the loss of its present one may succeed in driving out the inner being of an animal form and taking possession of it. If this mind is also very evil as well as deranged, it will utilize that form to terrorize a human community. But such happenings are breakaways from the ordinary processes of Nature and, therefore, uncommon. The penalty for such unnatural transmigration is insanity, which is the price which will have to be paid in the next human birth. The ego will then be tied to a body which it will be unable to use, yet unable to escape from.
-- Notebooks Category 9: From Birth to Rebirth > Chapter 2: Rebirth and Reincarnation > # 188